Microcircuits are well known electrical components that combine hundreds or thousands of individual circuit components and connections in a small volume. The package that holds a typical microcircuit may be no larger than 5 mm. square by 0.5 mm thick. One common type of container for a microcircuit called a leadless package, has small connector or contact pads along the periphery of one surface of the package. A single package may have several dozen contact pads by which power is supplied to the microcircuits and signals sent to and from the microcircuit. The contact pads are soldered onto the conductors of a circuit board during assembly of the electrical device.
Before a microcircuit is soldered onto a circuit board, the microcircuit must be tested to assure design functionality. Soldering a defective microcircuit onto a circuit board often ruins the entire board, since typically it is either not possible or not economic to remove a defective microcircuit from a circuit board. Since typical microcircuits are the result of a complex manufacturing process, testing is essential to assure that every microcircuit is completely functional.
For a number of reasons, testing these microcircuits is complex. In the first place, one should not solder the microcircuits to be tested into the test fixture because the act of removing the microcircuits when testing is complete might itself damage the microcircuit.
Secondly, the microcircuits are small and the contacts are closely spaced, on perhaps as small as a 0.3 mm pitch or smaller. The contacts themselves may be as small as 0.05 mm wide for accurate testing; the test fixture contacts must make reliable, low-resistance contact with each of the microcircuit contacts during the entire test process, which may extend to even many hours. Failure to make proper contact with each microcircuit contact for the entire test sequence results in a test that incorrectly fails the microcircuit.
While it is important to test each microcircuit thoroughly, it is also important to test them quickly and cheaply. Accordingly, automated testers have been developed that operate with little human intervention to reliably test hundreds or thousands of individual microcircuits per hour.
A typical tester has its own circuit board with one or more arrays of test contacts that are spaced and aligned to make temporary mechanical contact with the connector pads on the microcircuit package. Each test socket contact is designed to resiliently deflect a very small amount when force is applied. This accommodates any dimensional variations in either the microcircuit package or the test socket contacts.
An alignment plate is mounted on the tester circuit board with an aperture that receives and precisely positions each microcircuit to be tested so that each of the microcircuit contact pads is in precise alignment with the corresponding test contact. The alignment plate is typically bolted to the contactor which is mounted to tester circuit board.
To assure reliable and low resistance electrical conduction between each test socket contact and the corresponding microcircuit contact, the tester includes a presser or loader element that applies sufficient force to the microcircuit package so that each of the microcircuit package contacts at least slightly deflects the corresponding test socket contact. For example, if the test procedure requires 50 grams of force between each package contact and each tester contact, a package with 100 contacts will then require 5 kg. of force for proper electrical connection between each of the microcircuit contacts and the corresponding tester contact.
Some semiconductor devices operate at very high frequencies. In order to test them, higher performance contacts are needed. One method to improve the performance of a contact is to make it shorter and/or thinner. When the contact gets shorter, the housing must become thinner. This makes the housing more fragile and flexible. With a relatively thick housing, the contacts can easily be pre-loaded (i.e. applying a bias force to maintain the pin in one position) so that there is sufficient force and pressure to make reliable electrical connections to the load board and the Device Under Test (DUT). With a thin housing, pre-load force applied to the contacts may actually bend the housing and cause dimensional problems. If there is no pre-load on the contacts, it becomes difficult to produce sufficient forces on the contacts to make reliable electrical connections with the DUT and load board, so that is not a practical solution. As contacts get smaller for higher frequencies, the problem is exacerbated, because the pin doesn't move very far during compression and thus the pre-loading must be greater, that can distort (warp) the housing during insertion and test.